Color-television pictures are generally taken with a camera comprising respective tubes for the several color components, usually three tubes for the primary colors red, green and blue. These tubes may be individually adjusted for the purpose of yielding a well-balanced color picture, such adjustment being performed automatically with the aid of a multicolor test pattern whose color components are separately fed to the respective tubes. For this purpose it is necessary to dispose of a source of polychromatic light which either transluminates the test pattern or is episcopically reflected therefrom before being split into components of different wavelengths.
It has already been proposed, as described in U.S. Pat. No. 4,237,475, to synthesize the beam for the illumination of the test pattern from two or three differently colored ray bundles whose intensity can be individually adjusted in conformity with the desired color composition. The combination of the contributing ray bundles into a composite beam is carried out in a mixing device not described in detail in the patent.
Devices for additively or subtractively combining light rays of different colors are, of course, well known in the art. More often than not, however, light beams emitted by such devices lack the homogeneity which is needed for the proper adjustment of a color-television camera. The polychromatic beam required for the purpose here primarily contemplated should generally have a color composition varying by not more than 2% from its axis to its periphery. With conventional light mixers the deviation may be locally as high as 5% to 10%; proper uniformization necessitates in such a case the use of a complex and correspondingly expensive projection objective.
Another problem encountered with conventional illumination systems, especially those in which differently colored beams are emitted by sources in the form of incandescent lamps, resides in the lack of constancy of the relative and absolute intensity of the several color components due to warm-up of the filaments as well as to extraneous influences. The resulting variations in the measured parameters, occurring during a test period of only a few minutes, may lead to an incorrect setting of the camera controls.